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Activity 2 What is it?
CHEMICAL INTERACTIONS
Activity 2: What Is It?
1. Label each of the following statements as true (T) or false (F).
A. Density is the volume of equal masses of substances.
B. Different substances melt and boil at the same temperature.
C. You must observe or measure many properties of two unknown substances to
decide whether they are really the same or different.
D. Properties like electrical conductivity, density, and melting temperature are more
useful for identifying unknown substances than properties like shape, mass,
volume, and texture.
(Questions 2-6) Use the Table of Densities and the Table of Melting and Boiling
Points at the back of the book.
2. What gas is transparent, colorless and odorless; turns BTB yellow; does not burn;
dissolves in water; boils and condenses at a temperature of –78ºC; and has a density
of 1.84 g for each liter?
3. What gas is transparent, colorless and odorless; does not interact with BTB; does
not burn; boils and condenses at a temperature of –269ºC; and has a density of
0.18 g for each liter?
5. Which has a greater mass, a liter of copper or a liter of tin?
6. Imagine that you have three unknown substances: A, B, and C. The three substances
are solids consisting of small, white grains. These unknowns have two additional
properties shown in the data table below.
Table 2: Properties of Three Unknown Solids
Unknown A
Unknown B
Unknown C
Solid, white,
small grains
Solid, white,
small grains
Solid, white,
small grains
1 L has a mass
of 2650 g
1 L has a mass
of 1980 g
1 L has a mass
of 1680 g
1610ºC
776ºC
150ºC
2. Density
3. Melting point
1. a) False. Density is the mass of
equal volumes of substances.
b) True.
c) False. The first property tested
may indicate that the substances
are different.
d) True.
2. carbon dioxide
3. helium
4. Sugar (MP = 185˚C) melts at a
higher temperature than ice
(MP = 0˚C).
6. a) Solid A is sand (silicon dioxide)
because it is the only solid listed in
the Table of Melting and Boiling
Points with a melting point of
1610˚C. (The density of a liter
having a mass of 2650 g is not
listed in the Table of Densities, so
you must use the melting point to
identify this solid.)
Identify the three unknown solids. Justify your answers.
1. Sense properties
Activity 2 Practice
5. A liter of copper (mass = 8930 g)
has a higher mass than a liter of
tin (mass = 5750 g).
4. Which substance melts at a higher temperature, ice (solid water) or sugar?
Property
5
Unit 5 • Chapter 1
443
b) Solid B is potassium chloride
because it is the only solid listed in
the Table of Melting and Boiling
Points with a melting point of
776˚C. (The density of a liter
having a mass of 1980 g is not
listed in the Table of Densities, so
you must use the melting point to
identify this solid.)
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c) Solid C is Epsom Salts
(magnesium sulfate) because it is
the only solid listed in the Table of
Melting and Boiling Points with a
melting point of 150˚C. (The
density of a liter having a mass of
1680 g is not listed in the Table of
Densities, so you must use the
melting point to identify this solid.)
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Activity 3 Practice
1. Since the initial substances were
both liquid solutions, the
appearance of a white solid in a
different phase (at room
temperature) is good evidence for
a chemical reaction.
Activity 3: What’s the Evidence?
The White Solid
(Questions 1-2) In Unit 4 Chapter 2, Activity 2 you mixed a sodium sulfate solution
with a calcium chloride solution. You observed a white solid appearing in the beaker.
20 mL of
calcium
chloride
20 mL of
sodium
sulfate
2. Sodium sulfate reacts with calcium
chloride to produce calcium sulfate
and sodium chloride.
XLT
XLT
RESET
3. Since the initial substance was a
solid that does not dissolve in
water, and the ending substances
were a gas (different phase) and a
white powder that dissolves in
water, the differing properties of
these ending substances are good
evidence for a chemical reaction.
START MASS
RESET
END MASS
1. Explain why the appearance of the white solid is good evidence for a
chemical reaction.
The products of this reaction are calcium sulfate and sodium chloride. Calcium sulfate
does not dissolve in water, so it appears as a white solid. Sodium chloride is the
chemical name for the salt you sprinkle on your food. This salt stays dissolved in
the water.
2. Complete the following one-sentence description of this chemical reaction.
Sodium sulfate reacts with ________ to produce ________ and ________.
Heating Seashells
4. The reactant is calcium carbonate;
the products are calcium oxide and
carbon dioxide.
(Questions 3-5) Seashells are mostly made of the
substance called calcium carbonate. When seashells
are heated, a gas is released leaving a white powder
behind. This powder dissolves in water.
5. Calcium carbonate breaks down
(decomposes) into calcium oxide
and carbon dioxide.
3. What is the evidence that heating seashells results
in a chemical interaction?
Chemists have tested and identified the white powder
and the gas. The white powder is calcium oxide and
the gas is carbon dioxide.
4. What is the reactant in this chemical reaction?
What are the products?
5. Complete the following one-sentence description
of this chemical reaction.
________ breaks down (decomposes) into ________ and ________.
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Activity 4 Acid, Base, or Neutral?
CHEMICAL INTERACTIONS
Activity 4: Acid, Base, or Neutral?
1. Bromothymol blue (BTB) and litmus paper are examples of
a) indicators
b) strong acid
c) strong base
d) neutral salt
2. Indicators
a) are solutions with a red, blue, or yellow color.
b) form a yellow solid when they react with bases.
c) change color in an acide or base.
d) smell unpleasant when added to an acid.
3. How can you tell whether a food might contain an acid or a base as one of its
ingredients?
4. Suppose you added a nitric acid solution to a potassium hydroxide solution (a base).
What are the products of this acid-base (neutralization) reaction?
blue litmus paper
Write a one-sentence description of this chemical reaction.
(Questions 5-6) Acids turn blue litmus paper red, but they have no effect
on red litmus paper. Bases turn red litmus paper blue, but they have no
effect on the blue litmus paper. Neutral substances show no effect on either
red litmus paper or blue litmus paper (other than making the paper wet).
acid
red litmus paper
6. Blue litmus paper does not change color in an unknown substance. Is the unknown
substance an acid, neutral, base, or is it impossible to tell? Explain your reasoning.
The pH scale of the strength of acids and bases
Milk
of
Magnesia
apples
tomatoes
milk
ACID
0
gastric
juice
1
2
lemon
juice
3
4
orange
juice
5
6
clean
rain
NEUTRAL
7
8
9
baking
soda
milk of
magnesia
10
11
household
ammonia
2. (c) Indicators change color in an
acid or base.
3. If the food is a solid, you can try
dissolving it in water and drop the
solution into bromothymol blue and
yellow, or use an electronic pH
meter. If the food does not dissolve,
just wet it a little with some water,
and lay some litmus or Universalindicator paper on the moistened
food.
6. Even though you know the
substance is not an acid because it
did not turn blue litmus paper red,
you can’t tell if it is neutral or a
base.
oven
cleaner
12
1. (a) Bromothynol Blue (BTB) and
litmus paper are examples of
indicators.
5. The substance is a base, because it
turned red litmus paper blue.
(Questions 7-9) Use the pH scale below to answer the questions.
sea
water
Activity 4 Practice
4. In this neutralization reaction, nitric
acid (acid) reacts with potassium
hydroxide (base) to produce
potassium nitrate (a neutral salt)
and water (neutral).
base
5. Red litmus paper turns blue in an unknown substance. Is the unknown
substance an acid, neutral, base, or is it impossible to tell? Explain your reasoning.
battery
acid
5
BASE
13
14
drain
opener
ORANGE
ORANGE
JUICE
JUICE
Baking
Soda
7. Bananas are acidic, because their
pH number is lower than 7.
7. Bananas have a pH of about 5.1. Is a banana acidic, neutral, or basic? Explain
your answer.
8. This cola is acidic, because its pH
number is lower than 7.
8. A brand of cola has a pH of 2.4. Is this brand of cola acidic, neutral, or basic?
Explain your answer.
9. Which is the strongest base: baking soda, drain cleaner, household ammonia, or
milk of magnesia? Explain your answer.
445
9. Drain cleaner (pH of about 13.5)
is the strongest base among
household ammonia (pH of 11),
milk of magnesia (pH of 10.5),
and baking soda (pH of 8.5).
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Activity 5 Practice
1. a)
Activity 5: The Burning Reaction
2. d)
3. The reactants are glucose and
oxygen; the products are carbon
dioxide and water.
Multiple Choice
1. Which of the following is always a reactant in a burning reaction?
a) oxygen
b) soot
4. Glucose + oxygen ➞
carbon dioxide + water
c) carbon dioxide
d) water
5. Both burning methane and cellular
respiration (involving glucose) have
oxygen as a reactant and form the
products carbon dioxide and
water.
2. Which of the following is always a product in a burning reaction?
6. The reactants are lemon juice and
amines; the products are a salt and
water.
The rapid burning of fuel helps you survive cold weather, provides you with electricity,
and helps you move rapidly from place to place. But there is a slow interaction
involving oxygen that is even more important to your life. Your body is made up of
several hundred million cells. Where does the energy come from to make each of your
cells work? The energy comes from a chemical reaction involving oxygen. In each of
your cells, the sugar glucose combines with oxygen to produce carbon dioxide and
water, and releases energy for the cell. This process is called cellular respiration.
a) carbon monoxide
b) oxygen
c) fuel
d) water
Cellular Respiration
7. Lemon juice + amines ➞ salt +
water
3. What are the reactants of this chemical reaction? What are the products?
4. Write a word chemical equation for this reaction.
5. What is similar about burning methane and cellular respiration?
A Fishy Story
You know that citric fruit juices, including lemon juice, are acidic. Fish oils contain
dissolved bases called amines (uh-MEENS]. Amines do not smell good, but most people
think lemon juice smells good. Here is a practical example of an acid-base interaction.
lemon juice
amines
no fishy odor
6. What are the reactants? What are the products? (Hint: In Activity 4, you learned
the products of all acid-base reactions.)
7. Write a word chemical equation for this chemical interaction.
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Activity 6 Chemical Reactions and Mass
CHEMICAL INTERACTIONS
Activity 6: Chemical Reactions and Mass
Food for Plants
Where do you and other animals get the food you need to live and grow? From
plants! Green plants, algae, and some bacteria are the only organisms that make their
own food. The series of chemical reactions that makes food in plant leaves is called
photosynthesis (foht-OH-sinh-thuh-sihs). Plants use water from the roots and carbon
dioxide from the air to make a sugar called glucose. This reaction also produces
oxygen, which is released into the air.
Basic Photosynthesis
1. Write a word chemical equation for the
photosynthesis reaction.
light energy
oxygen
2. How does the mass of the reactants compare to the
mass of the products? Justify your answer.
sugar
carbon dioxide
Many people add liquid or solid
“plant food” to their garden and
houseplants. But plant food is not
really food for plants. Plants make
their own food — the sugar, glucose.
Plant foods contain substances such
as nitrogen, potassium, phosphorus,
and sulfur, that plants need for
healthy growth.
water
Instant Hot Packs
wrapper and pad
Chemical
Interaction
XLT
RESET
START MASS
XLT
rusting
RESET
Activity 6 Practice
1. Water + carbon dioxide ➞
glucose + oxygen
2. If you consider the system to be the
plant, this is an open system for
mass, with both mass inputs (water
and carbon dioxide) and a mass
output (oxygen). So, we cannot
compare the mass of the reactants
with the mass of the products
without knowing the actual values
of the mass inputs and outputs.
3. The rusting of iron reaction
requires oxygen as a reactant, so
the pad with iron must be removed
from the sealed wrapper and
exposed to the oxygen in air for
the reaction to occur.
4. The system of the wrapper and
pad is an open system for mass
because the reactant oxygen has to
come in from outside the system.
Since the product rust remains in
the system, this system only has a
mass input. Thus, the mass of the
wrapper and pad after several
hours will be greater than the mass
before opening.
Have you ever used an instant “hot pack” to reduce muscle pain or to keep your
hands warm in the winter? One type of hand warmer comes in a sealed wrapper.
Inside the wrapper is a soft mesh, cloth pad, which contains fine iron powder.
The iron rusting reaction lets the hand warmer stay at temperatures above 60ºC for
several hours.
unopened
5
PRACTICES—ANSWERS
MATERIALS AND THEIR INTERACTIONS
END MASS
3. Why do you have to open the sealed wrapper and take out the pad to start the
rusting reaction and the transfer of energy? (Hint: What is the other reactant in the
rusting reaction?)
4. Is the mass of the wrapper and pad after the pad has been removed for several
hours greater than, less than, or equal to the mass of the wrapper and pad before
it was opened? Justify your answer.
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Activity 7 Practice
1. The heating of seashells (calcium
carbonate) to produce carbon
dioxide gas and calcium oxide is
an endothermic reaction because
the calcium carbonate needs to
absorb (receive from surroundings)
energy for the reaction to occur.
Activity 7: Energy and Reactions
Heating Seashells
In the Practice for Activity 3, you learned that when seashells (calcium carbonate) are
heated, carbon dioxide gas is released, leaving solid calcium oxide. If you stop heating
the seashells, then the reaction stops.
1. Is this an exothermic reaction (that produces energy) or an endothermic reaction
(that absorbs energy)? Justify your answer.
2. Draw an energy diagram for this reaction.
2.
Source
Photosynthesis
Receiver
Surrounding
Objects
Energy transfer
Have you ever tried to grow a plant in a dark room or closet?
The plant shrivels up and dies. Without sunlight, the plant
cannot make its own food (the sugar glucose) in the
photosynthesis reaction (see the Practice for Activity 6).
System
Reactant (calcium carbonate)
and products (carbon dioxide
and calcium oxide)
3. Is photosynthesis an exothermic reaction or an
endothermic reaction? Justify your answer.
increases
in stored chemical
energy
4. Draw an energy diagram of the photosynthesis reaction.
3. The photosynthesis reaction is
endothermic because it requires
light energy from the sun to
produce its food, glucose.
4.
Inside a Car Engine
Source
Surrounding
Objects
Four-fifths of the air is nitrogen, which
usually does not take part in chemical
reactions. But at the very high temperatures
inside a car engine, the nitrogen reacts with
oxygen to produce the poisonous gas,
nitrogen oxide. The stored chemical energy
of the reactants (nitrogen and oxygen) is
greater than the stored chemical energy of
the product (nitrogen oxide).
Receiver
Energy transfer
Nitrogen reacts with
oxygen to produce
nitrogen monoxide
System
Reactants
(water and carbon dioxide)
and products
(glucose and oxygen)
increases
in stored chemical
energy
5. Is this an exothermic reaction or an endothermic reaction? Justify your answer.
(Hint: Use the reactant-product diagrams in the activity.)
6. Draw an energy diagram of the photosynthesis reaction.
5. The reaction inside the car engine
is exothermic. My reason is that the
stored chemical energy of the
reactants is greater than that of the
products, so energy must have been
transferred (output) from the system
to the surroundings.
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6.
Source
System
Reactants
(nitrogen and oxygen)
and product
(nitrogen oxide)
Receiver
Energy transfer
Surrounding
Objects
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decreases
in stored chemical
energy
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Activity 8 Describing Substances and Chemical Interactions
5
CHEMICAL INTERACTIONS
Activity 8: Describing Substances and Chemical Interactions
Multiple Choice
1. Which of the following is a chemical property of a substance?
a) It is a gas at room temperature.
b) It is flammable (it burns).
c) It has a density of 1.33 g/L of the gas.
d) It melts at a temperature of 218°C.
2. When a fuel (e.g., a candle, wood, or gasoline) reacts with the oxygen in the air,
the products are
a) a neutral salt and water.
b) the sugar, glucose and water.
c) calcium carbonate and water.
d) carbon dioxide and water.
PRACTICES—ANSWERS
MATERIALS AND THEIR INTERACTIONS
Activity 8 Practice
1. b)
2. d)
3. a)
4. c)
5. d)
6. d)
3. When an acid reacts with a base, one of the products is
a) a salt.
b) an indicator.
c) oxygen.
d) carbon dioxide.
4. In an exothermic reaction, the stored chemical energy of the reactants is
a) lower than the stored chemical energy of the products.
b) the same as the stored chemical energy of the products.
c) greater than the stored chemical energy of the products.
d) There is not enough information to tell.
5. When you mix hydrochloric acid with sodium hydroxide, two new substances are
produced: water and sodium chloride. Which of the following is the word chemical
equation for this reaction.
a) sodium chloride hydrochloric acid → water sodium hydroxide
b) sodium hydroxide → hydrochloric acid sodium chloride water
c) sodium chloride hydrochloric acid → water sodium hydroxide
d) sodium hydroxide hydrochloric acid → sodium chloride water
6. The pH scale indicates
a) the color an indicator changes in acids or bases.
b) the strength of an indicator.
c) whether the acid or base is a solid, liquid, or gas.
d) the strength of an acid or base.
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Activity 9 Practice
First Chemical Reaction:
1. Coke + oxygen ➞ carbon dioxide
Activity 9: Analyze and Explain
2.
iron ore
and coke
Source
Receiver
System
Reactants
(coke and oxygen)
and product
(carbon dioxide)
Blast
Furnace
exhaust gases
Surrounding
Objects
Energy transfer
One source of iron metal is the ore called hematite (iron oxide). To
obtain the iron from the hematite (iron oxide), the hematite is
heated with coke (a form of carbon) in a blast furnace. The furnace is
a large steel structure about 30 m high. It is lined with firebricks. A
mixture of the iron ore and coke is poured into the top of the
furnace. You will analyze the series of chemical reactions for
obtaining iron from hematite.
First Chemical Reaction: The coke (carbon) falls to the bottom
of the furnace to the level where the hot air blast (about 850ºC)
enters the furnace. The coke reacts with the oxygen in the air to
make carbon dioxide.
decreases
in stored chemical
energy
hot air blast
molten
slag
Second Chemical Reaction:
hot air blast
molten
iron
1. Write a word chemical equation for this reaction.
2. This reaction is exothermic. It raises the temperature of the blast
furnace to about 2000ºC. Draw an energy diagram for this
reaction.
Second Chemical Reaction: As the carbon dioxide rises in the
furnace, it reacts with more coke (carbon) to produce the poisonous
gas, carbon monoxide.
3. Coke + carbon dioxide ➞
carbon monoxide
3. Write a word chemical equation for this reaction.
4.
4. This reaction is endothermic. Draw an energy diagram for this reaction.
Source
Surrounding
Objects
Third Chemical Reaction: Finally, the iron oxide reacts with the carbon monoxide
gas to produce the metal iron and carbon dioxide gas.
Receiver
Energy transfer
5. Write a word chemical equation for this reaction.
System
Reactants
(coke and carbon dioxide)
and products
(carbon monoxide)
6. The stored chemical energy of the products is greater than the stored chemical
energy of the reactants. Is this reaction exothermic or endothermic? Justify your
answer.
The iron, which is molten at the high blast furnace temperature, trickles down to the
base of the blast furnace, where it is removed. The iron is cooled and solidifies into pig
iron or is transferred directly to a steel-producing furnace. The waste impurities form a
molten liquid is called slag. When the slag is removed and cooled, it solidifies and can
be used in road building.
increases
in stored chemical
energy
Third Chemical Reaction:
5. iron oxide + carbon monoxide ➞
iron + carbon dioxide
Three forms of iron ore (hematite).
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6. The reaction is endothermic. My
evidence is the stored chemical
energy of the products is greater
than the stored chemical energy of
the reactants, so energy must have
been transferred (input) from the
surroundings to the system.
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SCIENTISTS’ CONSENSUS IDEAS—ANSWERS
Scientists’ Consensus Ideas
UNIT 5 CHAPTER 1 Answer Keys
Idea 1. Materials and Properties
1. a. Chemists describe materials by their properties. A property of a material is a description of
how the material interacts with other things, like our senses or instruments that extend our
senses.)
b. Different materials are identified by a unique set of properties.
c. Characteristic properties are more useful for identifying unknown substances than
properties like shape, mass, volume, or texture.
Examples of Characteristic Properties:
Density, melting and boiling points, electrical conductivity
Idea 2. Chemical Properties
2. Chemical properties of a substance are descriptions or measurements of how a material
interacts chemically with other materials.
Examples of Chemical Properties:
• Whether or not the substance supports burning
• whether or not the substance itself burns (flammability, interaction with a flame)
• interaction that makes the substance (what reactants are involved)
• interaction with indicators (e.g., BTB, litmus paper)
• measurement of pH number
• reaction with some chemicals (e.g., water, acids, oxygen)
Idea 3. Classifications of Materials
Chemists find it useful to identify classes of substances with common properties
3. a. Acids taste sour, react with certain indicators to produce a new substance with a different
color, react with certain metals to produce hydrogen gas, and react with a base to produce a
neutral salt and water. (Act 4)
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Examples of Some Acids:
apple juice (malic acid), lemon juice (citric acid), orange juice (ascorbic acid or vitamin
C), vinegar (acetic acid), battery (sulfuric) acid, gastric juice (hydrochloric acid)
Examples of Some Indicators for Acids:
BTB (turns yellow when reacted with an acid)
blue litmus paper (turns red when reacted with an acid)
universal-indicator paper or pH meter
Examples of Some Metals That React With Acids:
magnesium, aluminum, iron, zinc
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b. Bases taste bitter, feel slippery, react with certain indicators to produce a new substance
with a different color, do not react with metals, and react with acids to produce a neutral
salt and water (U5C1 Act 4)
Examples of Some Bases:
baking soda (sodium bicarbonate), window cleaner (ammonia), tonic water (quinine), drain opener
(sodium hydroxide), milk of magnesia (magnesium hydroxide), calcium oxide (lime), calcium hydroxide
(slaked lime)
Examples of Some Indicators for Bases:
bromothymol yellow (turns blue when reacted with a base)
red litmus paper (turns blue when reacted with a base)
universal-indicator paper or pH meter (indicates a pH number)
c. Neutral substances are neither acids nor bases. They usually do not change the color of
acid-base indicators. (U5C1 Act 4)
Examples of Some Neutral Substances:
Distilled water, sugar, salt, milk, alcohols, many fuels
d. Salts are neutral substances that are one of the products of an acid-base (neutralization)
chemical reaction. (U5C1 Act 4)
Examples of Some Salts:
Sodium acetate, potassium chloride, sodium chloride (table salt), Epsom salt (magnesium sulfate)
e. Fuels are substances that release large amounts of heat energy when they burn (react with
oxygen). (U5C1 Act 4)
Examples of Some Fuels:
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Kerosene, gasoline, jet fuel, methanol
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DEFINING CHARACTERISTICS
OF
SCIENTISTS’ CONSENSUS IDEAS—ANSWERS
Scientists’ Consensus Ideas
UNIT 5 CHAPTER 1 Answer Keys
CHEMICAL INTERACTIONS
Idea 1. A chemical interaction is any type of interaction (such as mechanical, heat
conduction, or electric current) that results in at least one new material. (Act. 3)
Names of Common Chemical Interactions:
baking, roasting, broiling, rotting, rusting, burning, exploding, digesting food, breaking-down,
composting.
Idea 2. The evidence of a chemical interaction is that the materials at the end of the
interaction have a different set of properties than the original materials (before the interaction).
Observations that can provide evidence of a chemical interaction include:
A. The disappearance of one of the original materials. (Act. 2)
Examples:
crushed chalk and vinegar (some of the chalk dust disappears)
baking soda and vinegar (some of the baking soda disappears)
rusting (some of the oxygen in the air disappears)
burning (some of the oxygen in the air disappears)
compost (some of the decaying material disappears)
B. The appearance of a material in a different phase (at room temperature). (Act. 3)
Examples:
crushed chalk and vinegar (a gas appears from a solid and a liquid)
baking soda and vinegar (a gas appears from a solid and a liquid)
milk and lemon juice (a white solid appears from two liquids)
burning candle in a closed jar (water and black soot appears on the inside of the jar)
C. Changes in other properties (e.g., color, odor, texture, density, hardness, size or shape, interaction with
water (dissolves, does not dissolve) flammability (burns or does not burn in air), reaction with acid-base
indicators, and so on). (Act. 3, Act. 5, Act. 6)
Examples:
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bubbling carbon dioxide through BTB solution changes its color to yellow
crushed chalk and vinegar (an odorless, colorless gas appears from a odorless white solid and a
smelly colorless liquid)
match and air (a black solid appears from match’s red tip and light brown stem)
bread dough and baked bread (The outside crust of the bread is harder and a darker brown color
than the original bread dough. The baked bread is larger than the dough (change in size). The
dough feels wet and sticky, is kind of elastic (you can stretch it), and is thick. The inside of baked
bread is dry and not sticky, tears apart when you pull it, and has lots of holes (is not thick.)
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Examples:
Mixing vinegar and baking soda in a closed baggie:
acetic acid mass + sodium bicarbonate mass ➝ carbon dioxide mass + sodium acetate mass +
water mass
Rusting iron in a sealed flask:
iron mass + oxygen mass ➝ rust mass
Idea 5. Chemical Reactions and the
Conservation of Energy
A. Once started, all chemical reactions either produce energy–continually transfer energy out of the system of
interacting substances to the surrounding objects or absorb energy–require a continual transfer of energy
into the system of interacting substances.
Chemists call reactions that produce (release) energy exothermic. All reactions that absorb energy
Exothermic
(require continual energy input) are called endothermic.
chemical energy of the reactants is greater than the stored chemical
energy of the products.
So for energy to be conserved, energy is transferred out of the
system and the stored chemical energy of the system decreases
Reactants
decrease
stored chemical energy
B. A chemical reaction is exothermic (produces energy) when the stored
Products
Examples of Exothermic Reactions:
• burning methane gas
System
Reactant and Product Substances
Energy transfer
Energy
Output
Surrounding
Objects
• rusting iron
• reaction of nitrogen and oxygen in
hot car engine
decrease
in stored chemical
energy
chemical energy of the products is greater than the stored chemical
energy of the reactants.
So for energy to be conserved, energy is transferred into the system
and the stored chemical energy of the system increases.
Surrounding
Objects
Energy transfer
Products
Energy
Output
increase
C. A chemical reaction is endothermic (absorbs energy) when the stored
stored chemical energy
Endothermic
Reactants
System
Reactant and Product Substances
Examples of Endothermic Reactions:
• baking soda (sodium bicarbonate) and vinegar (acetic acid)
• (video of reacting) barium hydroxide and ammonium thiocyanate
• photosynthesis
• heating sea shells (calcium carbonate)
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increase
in stored chemical
energy
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SCIENTISTS’ CONSENSUS IDEAS—ANSWERS
© It’s About Time
NOTES
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